how can I render Smoke&fire in Cycles 2.79?
Firstly. Your nodes:


This setup is by no means perfect but it should work. Attach it to your smoke simulation domain.

If that doesn't work, try placing down a single circle, filled. Go to:

Objects --> Quick Effects --> Quick Smoke.

Play animation...

If you render and it works, then there is something wrong with your nodes/object. I recommend trying to recreate your previous attempt.
As annoying as it may seem, you should have a better idea on how to recreate it and will replicate it faster.

If it doesn't work there may be something you have changed in your render settings. Try doing the Quick Smoke method on a new document and see if it works. I still have much learn about blender myself so I can't help you there, sorry.
There shouldn't be any issues with GPU rendering in 2.79.

Good luck, Hope I helped,

BFB